1. Field of the Invention
The present invention generally relates to a magnetic sensor. More specifically, the invention is directed to a magnetic sensor suitable for detecting a magnetic field produced by a moving magnetic medium.
2. Description of Prior Art
In a prior art magnetic sensor for detecting a magnetic field produced by the movement of the magnetic medium, as illustrated in FIG. 4, the magnetic sensor 12 is positioned opposite to a magnetic encoder type magnetic drum 11, an amplifier 13 is connected to an output terminal of this magnetic sensor 12, and a waveform shaping circuit 14 is connected to an output terminal of the amplifier 13. Based upon an output signal from this waveform shaping circuit 14, an amount of rotation as well as a direction of rotation of the magnetic drum 11 are detected.
In this case, the structure of the magnetic sensor 12 is illustrated in FIG. 5. When InSb is employed as magnetic resistance elements R.sub.1 to R.sub.4, a thin sliced piece is attached to a glass piece, whereby a thin film having a thickness of 5 to 10 .mu.m is formed by way of a mechanical or chemical method, and a metal electrode is inserted therein by a photo-lithography means.
In case that Fe-Ni as a ferromagnet is employed for the magnetic resistance elements R.sub.1 to R.sub.4 which is a major magnetic material, the magnetic resistance element having a thickness of 200 to 1000 .ANG. is formed on a substrate surface such as a glass piece by a sputtering method, and a metal electrode for a lead wire is similarly formed, which is similar to the above-described conventional method.
In FIG. 5, a metal electrode 1 is used as an electrode for applying a bias voltage Vc, a metal electrode 2 is utilized as a ground electrode, metal electrodes 3 and 4 are employed as signal output terminals, whereby overall metal electrodes constitute a bridge circuit. A repetition period (S-N-S) of a magnetic field variation which is produced by the rotation of the magnetic drum 11 as a magnetic medium is determined as .lambda., and the magnetic resistance elements R.sub.1 to R.sub.4 of a ferromagnet are positioned with a 1/4-.lambda. positional shift. Although FIG. 5 shows only the A-phase detecting circuit portion, there is also a B-phase detecting circuit (not shown) having the same circuit arrangement as that of this A-phase detecting circuit. This B-phase detecting circuit is positional-shifted by 7/8.multidot..lambda. with respect to the A-phase detecting circuit.
The conventional magnetic sensor 12 having such an arrangement is positioned opposite to the magnetic drum 11 of the magnetic medium, as illustrated in FIG. 4, and resistance values of the magnetic resistance elements R.sub.1 to R.sub.4 are varied since the magnetic pole pattern S-N-S-N formed on the magnetic drum 11 alternately approaches these magnetic resistance elements R.sub.1 to R.sub.4 while the magnetic drum 11 is rotated, thereby changing the magnetic field.
When this resistance value change is caused by the magnetic resistance elements R.sub.1 to R.sub.4 of InSb, the magnetic field produced by the magnetic medium, which intersecting with the magnetic resistance elements R.sub.1 to R.sub.4 under application of the bias voltage Vc, is applied, and a Hall effect is produced under an in-phase relationship with this magnetic field variation, and then an internal impedance is varied. In this case, the magnetic resistance elements R.sub.1 to R.sub.4 are positionally shifted by 1/2.lambda. with each other. It should be noted that although FIG. 5 shows only the A-phase detecting circuit portion, there is also provided the B-phase detecting circuit having the same circuit arrangement as that of the A-phase detecting circuit is positionally shifted by 3/4.lambda. with respect to the A-phase detecting circuit.
When this resistance value variation is caused by the magnetic resistance elements R.sub.1 to R.sub.4 made of the ferromagnet of Fe-Ni, the direction of the self-exciting magnetization in the elements which is produced in the biasing current direction is inclined due to the magnetic field perpendicular to the biasing current direction by the magnetic medium, whereby the electric resistance value is varied at a two times higher period than that of this magnetic field variation.
As previously described, the magnetic resistance value variation mode of the magnetic resistance elements which are positionally shifted by 1/4.lambda. is the reverse phase, whereas that of the magnetic resistance elements which are positionally shifted by 7/8.lambda. is the 90-degree phase.
Thus, based on the output signals having the reverse phase with each other and derived from the metal electrodes 3 and 4 as the signal output terminals shown in FIG. 5, the magnetic field produced by the magnetic medium is detected so as to detect the amount of rotation and also the rotation direction of the magnetic medium.
As represented by a solid line of FIG. 6, the above-described magnetic resistance elements R.sub.1 to R.sub.4 are so constructed that a length "l" of the magnetic medium is 3 mm and a width "t" thereof is 20 .mu.m in the direction normal to the moving direction as denoted by an arrow X.
If the magnetic resistance elements R.sub.1 to R.sub.4 are formed in such a manner that they are inclined in the direction perpendicular to the moving direction of the magnetic medium, like the magnetic element R.sub.1 of FIG. 6, the magnetic field produced by the magnetic medium cannot be correctly detected. In case that, for instance, the magnetic pole pattern pitch P(S-N) of the magnetic medium is 80 .mu.m, the allowable azimuth angle of the magnetic resistance elements R.sub.1 to R.sub.4 will be defined by the following equation, using the above-described l, t, and P. ##EQU1##
In the above equation (1), assuming that "l" is selected to be 3 mm, "t" is selected to be 20 .mu.m and "P" is selected to be 80 .mu.m, the allowable azimuth angle .theta., is equal to 0.2.degree.. Accordingly, the magnetic resistance elements R.sub.1 to R.sub.4 are required to be inclined within the angle of 0.2 degrees. Since such an inclined angle requires the highly manufacturing precision of the magnetic sensor, the mass production thereof is necessarily lowered.
In this case, if the magnetic pole pattern pitch of the magnetic medium is set to be large, and also the shapes of the magnetic resistance elements R.sub.1 to R.sub.4 are made large, the lower manufacturing precision is allowed. However, as a compact magnetic resistance element is normally required recently, this solution can be hardly accepted.
The present invention has been made to solve the above-described conventional problems of the magnetic sensor, and therefore has an object to provide a compact magnetic sensor wherein the allowable azimuth angle of the magnetic resistance element can be readily relaxed.